KENNEDY SPACE CENTER, Fla. (CBS) - NASA remains on track for launching the shuttle Discovery on the first post-Columbia mission next May or June, senior agency managers said today. While certified techniques for on-orbit repair of tile or wing leading edge damage may not be available by then, efforts to minimize foam debris impacts, coupled with a variety of other safety upgrades and the "safe haven" provided by the space station, give managers confidence the shuttle can safely return to flight sometime next spring.

The next available launch window for mission STS-114 opens May 12 and closes June 3. Engineers at the Kennedy Space Center currently are in the process of stacking the ship's twin solid-fuel boosters and installing three hydrogen-fueled main engines. Discovery's external fuel tank, the subject of extensive insulation design changes in the wake of the 2003 Columbia disaster, is scheduled to arrive at the spaceport around Jan. 5.

Columbia was destroyed because of wing leading edge damage caused by a chunk of insulating foam that broke off the ship's external fuel tank during launch. During entry, hot gas ate its way into the left wing's interior, triggering structural failure 37 miles above Texas.

The foam in question, used to insulate the fittings that help attach a shuttle's nose to the tank, has been removed and replaced with heaters to prevent dangerous ice buildups. Other changes have been implemented to minimize roam shedding from other areas of the huge tank.

"I think we have done a very thorough job of working our way through the understanding of how debris is liberated, understanding what sizes will come off, how that debris is transported through the air stream and how it might impact the orbiter," William Parsons, shuttle program manager at the Johnson Space Center in Houston, told reporters today. "So I'm feeling very confident we have our arms around this.

"But what we have to do is, we have to verify that. And the best way to verify that is to fly a mission. So we're going to go fly a couple of test missions and we're going to take a lot of imagery and we're going to do a lot of inspections and we're going to have sensors in place so that we can verify what we feel very confident that we've accomplished."

Wayne Hale, a former flight director who now serves as Parsons' deputy and chairman of the mission management team, said today the requirement is for nothing to come off the tank that weighs more than 0.03 pounds.

"All our investigations of the foam indicate we will not get a piece of foam coming off bigger than .008, eight one thousandths (of a pound), or almost an order of magnitude smaller than the requirement," he said. "Now, does that mean we won't see a chip in a tile here or there? I would say we'd be optimistic if we said that.

"But we're clearly moving toward an area where we expect to see much less damage in the tile and no critical damage that will require a repair. So that's our goal in this and it's beginning to look very positive that we'll be able to accomplish that level of control on the ET foam."

Parsons said engineers are still wrestling with one aspect of the tank's redesigned insulation - an area around a bellows in a liquid oxygen feed line - but he is optimistic the tank will ship on time. He said any additional work in that area can be performed at Kennedy.

Along with minimizing potential debris, NASA has installed sensors behind the shuttle's wing leading edge panels that will detect any significant impacts. Upgraded cameras on the ground and aboard the shuttle will document whatever foam shedding does occur and the astronauts will carry out an exhaustive leading edge inspection in space using cameras and laser sensors mounted on a long boom.

But NASA is struggling to perfect techniques for spacewalking astronauts to repair tile or leading edge damage in space in the event such damage actually occurs.

Agency managers earlier said certified repair techniques for reinforced carbon carbon leading edge panels likely would not be available in time for STS-114. Such repair techniques were not considered a requirement for flight. That was due in large part to a new capability for the crew of a stricken shuttle to use the space station as a "safe haven" until a rescue flight could be launched.

Today, Parsons and Hale said the same philosophy will apply to tile repair. While engineers are continuing efforts to perfect repair techniques, and while two astronauts will carry out tests of those procedures, a certified methodology is not a pre-launch requirement for STS-114.

"When we started this, we knew it would be extremely difficult to do a repair technique both on tile and on RCC," Parsons said. "I think we said that from the very beginning. The CAIB (Columbia Accident Investigation Board) said to do the best that we could to come up with some technique to repair RCC and repair tile. We have done that. We have continued to put the best and brightest this agency has to offer on this, we've used every resource this agency and this nation has to work on this, we've made a lot of progress."

In recent weeks, engineers have raised questions about the possibility of air bubbles forming in the tile repair material that could degrade its performance during entry.

"We think there are some issues that we still need to resolve, but we still have some time to go resolve that," Parsons said. "And then we're going to go fly a test mission. We're going to have a detailed (repair demonstration spacewalk) in the payload bay of the orbiter. ... We can do a lot of things on the ground, but we have to take them on orbit, put them in work on orbit, bring them back and then see if they work as well as we think they'll work.

"In a state of emergency, we would have a technique that we would be ready to perform (during STS-114)," he said. "But right at this moment, we're still working through some of the technical details on how to do good tile repair and RCC repair."

Pressed to say whether on-orbit repair capability of both RCC and tile were, or were not, launch constraints, Hale said "The Columbia Accident Investigation Board did not say it was a requirement."

Parsons added, "but they did say to work on it and they did say to do the best that we could and I think what Wayne's telling you is we have done the best that we could. We have made a lot of progress and we think we'll have something there that we can use and test on STS-114."

For the record, the Columbia Accident Investigation Board said that for missions to the international space station, NASA should "develop a practicable capability to inspect and effect emergency repairs to the widest possible range of damage to the thermal protection system, including both tile and reinforced carbon carbon, taking advantage of the additional capabilities available when near to or docked at the international space station."

Hale stressed that NASA will not be driven by target launch dates and that Discovery will not fly until the agency's management team is convinced it is safe to do so.

"We are going to fly when we have determined that the vehicle is ready to fly, when it is safe to fly," he stressed. "We're not being driven by a calendar date, we're being driven by our readiness to go fly. So when we are convinced the external tank is in a good situation, when we are convinced we have the warning devices, the OBSS (inspection boom) and all those other things, wing leading edge sensors, all installed, checked out and ready to go, when we are convinced we have an adequate repair capability, then we'll go fly.

"The date will kind of be what the date is," he added. "We're not going to succumb to some kind of emotional schedule."

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